The present invention relates to a moistureproof film having high moistureproof performance, and more particularly to a moistureproof film suitable for use as a packaging film for sealing an electroluminescence device, and a production process thereof. The present invention also relates to an electroluminescence device sealed with this moistureproof film and a fabrication process thereof.
An action that an electric field is applied to a solid luminescent compound (phosphor) to convert electric energy into luminescent energy is referred to as electroluminescence (EL). EL can be classified into a thin film type and a dispersion type from the basic device structure thereof. The thin film type EL device comprises a luminescent layer composed of a thin film of a phosphor. The dispersion type EL device comprises a luminescent layer in which a powdery phosphor is dispersed in an organic or inorganic binder. An EL device comprises a device body with a luminescent layer held between a pair of electrodes directly or through an insulating layer, and a transparent electrode is used as at least one of the pair of electrodes. The EL device is divided into a DC drive type (DC type) and an AC drive type (AC type) according to whether drive voltage is direct current or alternating current.
The uses of the EL device are about to spread as, for example, a back light for liquid crystal display devices, a surface illuminant for all-night lights, road traffic signs, night advertisement, decorations and the like, or a flat display such as a terminal display used in computers or word processors, or an image displays used in televisions, making good use of its features that it is thin in film thickness and light in weight.
When the phosphor forming the luminescent layer absorbs moisture or water vapor, the luminance thereof is impaired. Therefore, the EL device generally has a structure that a device body, in which the luminescent layer has been arranged between a pair of electrodes, has been sealed with a transparent moistureproof material. As the moistureproof material for the EL device, there has heretofore been used a moistureproof film composed mainly of a poly(chlorotrifluoroethylene) (PCTFE), or a glass base. Of these, the glass base has a limit to the formation of a thin film or weight reduction and also involves a problem that it is lacking in flexibility. On the other hand, PCTFE is high in cost because it is a fluorocarbon resin. Besides, the moistureproof performance of the PCTFE film is markedly deteriorated when atmospheric temperature exceeds 50xc2x0 C. Therefore, the PCTFE film has involved a problem that the life of an EL device making use of such a film is extremely shortened at high temperatures. In addition, there is a reason that the access to raw materials for fluorocarbon resins is expected to be hard in future. There has thus been a demand for development of another resin material substitutable for PCTFE.
As such other resin materials, polyvinylidene chloride and polyvinyl alcohol have heretofore been investigated. Resin films, on which a vapor-deposited silicon oxide layer has been provided, have also been investigated. However, all these materials are insufficient in moistureproof performance compared with the case where the PCTFE film is used, and fail to lengthen the life of an EL device. Therefore, they do not come to be put to practical use.
It is an object of the present invention to provide a moistureproof film which has moistureproof performance to the degree equal to or higher than the conventional moistureproof film composed mainly of poly(chlorotrifluoroethylene) film, exhibits moistureproof performance stable to changes in environmental temperature and humidity, may be formed in a thin and light-weight film and is suitable for use in sealing an EL device.
Another object of the present invention is to provide a production process of the moistureproof film having such high moistureproof performance.
A further object of the present invention is to provide an EL device sealed with the moistureproof film having such high moistureproof performance and a fabrication process thereof.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. In the course thereof, attention has been paid to a polyvinyl alcohol film (hereinafter abbreviated as xe2x80x9cPVA filmxe2x80x9d) on which a vapor-deposited layer composed of a metal or nonmetal oxide typified by a thin silicon oxide layer was formed. The PVA film with such a vapor-deposited layer formed thereon (hereinafter abbreviated as xe2x80x9cvapor-deposition PVA film) is transparent and extremely low in water vapor transmission rate which can be used as a measure of water vapor barrier property. However, the vapor-deposition PVA film is poor in practical moistureproof performance and is hence not used effectively as a moistureproof film for packaging an EL device unlike the PCTFE film.
In fact, the present inventors have used a multi-layer film comprising the vapor-deposition PVA film has been used as a moistureproof film to seal an EL device. As a result, it has been found that the luminance of the EL device is markedly deteriorated when the EL device is exposed to high-humidity conditions. The present inventors have carried out a further investigation as to the reason why only a moistureproof film poor in practical performance can be provided though the multi-layer film comprising the vapor-deposition PVA film has an extremely low water vapor transmission rate in itself. As a result, it has been presumed that in the vapor-deposition PVA film or the multi-layer film comprising the vapor-deposition PVA film, the moisture absorption of PVA, which is a hygroscopic resin, is unavoidable under ordinary conditions for production, and so a PVA film contains some water, and that fact may adversely affect the practical performance.
It has thus been surprisingly found that when a multi-layer film of a structure that thin silicon oxide layers are respectively arranged directly or through an adhesive layer on both sides of a PVA film is used, and the PVA film, vapor-deposition PVA film or multi-layer film used is thoroughly dried to a substantially absolute dry state, a novel moistureproof film which has moistureproof performance to the degree equal to or higher than the moistureproof film composed mainly of PCTFE film can be provided.
The present inventors have found that the moistureproof performance of the vapor-deposition PVA film or the multi-layer film comprising the vapor-deposition PVA film cannot be evaluated simply by the water vapor transmission rate thereof and should be evaluated by tempering the water vapor transmission rate with the quantity of water vapor transmission (the measuring method thereof will be described subsequently). Multi-layer films of the structure that thin silicon oxide layers are respectively arranged directly or through an adhesive layer on both sides of a PVA film, one of which was produced under ordinary conditions for production and the other of which was thoroughly dried in accordance with the production process of the present invention, have been separately used as a moistureproof film to fabricate respective EL devices, and the quantities of water vapor transmission in such devices have been actually measured. As a result, it has been found that there is an extremely great difference in the quantity of water vapor transmission of the moistureproof films between both devices. In the EL device making use of the multi-layer film produced under the ordinary conditions for production, the retention of luminance is greatly lowered when it is exposed to a high-humidity environment, and the luminance may be lost in an extreme case. On the other hand, the use of the moistureproof film according to the present invention, comprising the multi-layer film thoroughly dried permits the provision of an EL device which exhibits high retention of luminance over a long period of time even when it is exposed to a high-humidity environment.
The reason why the moistureproof film according to the present invention exhibits such high moistureproof performance, or the mechanism thereof is not always wholly clarified at the present stage. However, the present inventors are considered to be as follows. The conventional vapor-deposition PVA film absorbs moisture under ordinary conditions for production and hence contains some water. This is called xe2x80x9cinitial water absorptionxe2x80x9d. The water vapor transmission rate of the vapor-deposition PVA film or the multi-layer film comprising the vapor-deposition PVA film is not very affected by the initial water absorption, but the quantity of water vapor transmission is greatly affected thereby. If the quantity of water vapor transmission is great, only a moistureproof film poor in practical moistureproof performance can be provided. On the other hand, when the vapor-deposition PVA film or the multi-layer film comprising the vapor-deposition PVA film is thoroughly dried, the initial water absorption is lowered, and moreover the resistance to water vapor transmission of the PVA film itself is markedly improved by the aging effect and the like. As will hereinafter be described, it is further presumed that the improvement of the resistance to water vapor transmission by the vapor-deposition layer and adhesive layers may contribute to the improvement of moistureproof performance. Such an effect of markedly improving the moistureproof performance by the drying treatment is too marked to be anticipated by the prior art.
The multi-layer film of the structure that the thin metal or nonmetal oxide layers are respectively arranged directly or through an adhesive layer on both sides of the PVA film exhibits-an excellently low water vapor transmission rate. However, the desired dry state of the interposed PVA film (typical hygroscopic resin layer) and in its turn the multi-layer film cannot be simply achieved due to the obstruction by the water vapor barrier performance inherent in the thin oxide layers present on both sides. However, once the desired dry state is achieved by thoroughly conducting a drying treatment at high temperature for at least 10 hours, or over at least 100 hours if circumstances require, a moistureproof film, which can stably retain an excellently low quantity of water vapor transmission, can be provided in cooperation with its low water vapor transmission rate.
The moistureproof film according to the present invention undergoes no adverse influence on its moistureproof performance even by changes in environmental temperature and humidity. It is also possible to impart flexibility to the film by suitably selecting the layer structure thereof. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a moistureproof film composed of a transparent multi-layer film having a layer structure that thin metal or nonmetal oxide layers (C) are respectively arranged directly or through an adhesive layer (B) on both sides of a hygroscopic resin layer (A), wherein the moistureproof film has:
(1) a water vapor transmission rate of at most 0.05 g/m2xc2x724 hr as measured at a temperature of 60xc2x0 C. and a relative humidity of 90%; and
(2) a quantity of water vapor transmission for 50 hours of at most 0.15 g/m2 as measured at a temperature of 40xc2x0 C. and a relative humidity of 100%.
According to the present invention, there is also provided a process for producing a moistureproof film composed of a transparent multi-layer film having a layer structure that thin metal or nonmetal oxide layers (C) are respectively arranged directly or through an adhesive layer (B) on both sides of a hygroscopic resin layer (A), the process comprising the step of drying the multi-layer film or the hygroscopic resin layer (A) making up the multi-layer film, thereby forming a multi-layer film having:
(1) a water vapor transmission rate of at most 0.05 g/m2xc2x724 hr as measured at a temperature of 60xc2x0 C. and a relative humidity of 90%; and
(2) a quantity of water vapor transmission for 50 hours of at most 0.15 g/m2 as measured at a temperature of 40xc2x0 C. and a relative humidity of 100%.
According to the present invention, there is further provided an electroluminescence device comprising a body of the electroluminescence device, in which a luminescent layer has been arranged between a pair of electrodes, and which has been sealed with a moistureproof material at least a part of which is composed of a moistureproof film, wherein the moistureproof film is a moistureproof film composed of a transparent multi-layer film having a layer structure that thin metal or nonmetal oxide layers (C) are respectively arranged directly or through an adhesive layer (B) on both sides of a hygroscopic resin layer (A), and having:
(1) a water vapor transmission rate of at most 0.05 g/m2xc2x724 hr as measured at a temperature of 60xc2x0 C. and a relative humidity of 90%; and
(2) a quantity of water vapor transmission for 50 hours of at most 0.15 g/m2 as measured at a temperature of 40xc2x0 C. and a relative humidity of 100%.
According to the present invention, there is still further provided a process for fabricating an electroluminescence device comprising a body of the electroluminescence device, in which a luminescent layer has been arranged between a pair of electrodes, and which has been sealed with a moistureproof material at least a part of which is composed of a moistureproof film, the process comprising using, as the moistureproof film, a transparent multi-layer film having a layer structure that thin metal or nonmetal oxide layers (C) are respectively arranged directly or through an adhesive layer (B) on both sides of a hygroscopic resin layer (A), and the step of drying the multi-layer film or the hygroscopic resin layer (A) making up the multi-layer film in such a manner that the moistureproof film has:
(1) a water vapor transmission rate of at most 0.05 g/m2xc2x724 hr as measured at a temperature of 60xc2x0 C. and a relative humidity of 90%; and
(2) a quantity of water vapor transmission for 50 hours of at most 0.15 g/m2 as measured at a temperature of 40xc2x0 C. and a relative humidity of 100%.